Field of the Invention
Embodiments of the present disclosure relate to a method and apparatus of delivering a high quality audio output from an audio device that is powered by a power source that has a limited energy storage capacity.
Description of the Related Art
The popularity of portable music players has increased dramatically in the past decade. Modern portable music players allow music enthusiasts to listen to music in a wide variety of different environments without requiring access to a wired power source. For example, a battery-operated portable music player such as an iPod® or a wireless speaker coupled to an iPod® or similar device is capable of playing music in a wide variety of locations without needing to be plugged in. Conventional portable music players and/or wireless speakers are typically designed to have a small form factor in order to increase portability. Accordingly, the batteries within such devices are usually small and only provide several hours of operation before the batteries need to be recharged or replaced.
As a result, the speakers within conventional portable music players and conventional wireless speakers often times have a dynamic range covering only a fraction of the frequency spectrum associated with most modern music. For example, modern music often includes a wide range of bass frequencies. However, the speakers within a conventional portable music player or wireless speaker usually cannot play all of the bass frequencies due to physical limitations of the speakers themselves, or because of the way the amplifying circuitry in the portable music player or wireless speaker are driven causes the useable power found in the batteries within the device to discharge rapidly. The power supply in conventional portable music players and wireless speakers is commonly limited by a finite energy storage capacity provided by the battery. The rate of energy consumption by the device determines the time of operation of the device until the battery needs to be recharged or replaced.
A power amplifier in an audio device receives an input signal, and using a power supply voltage, produces an output signal having the same shape but larger magnitude than the input signal. Typical conventional audio devices use linear types of power amplifiers, such as class A and AB amplifiers, due to simplicity and in most cases desirable sound quality is provided using these types of amplifiers. The efficiency of most of these analog amplifiers is poor, thus reducing the time of operation of the audio delivery device between charges of a battery-type power supply.
Power amplifiers are used in the output stages of audio devices to drive a loudspeaker load. Typical loudspeakers may have nominal impedance in a range between about 4 ohms (Ω) and 8Ω, but the actual load impedance varies with frequency. Power amplifiers must be able to supply the high peak currents and peak voltages required to drive these loudspeakers and operate efficiently with a dynamic range of voltage amplitude as part of audio input signal data. The “rail” voltage provided to the amplifier needs to deliver sufficient power to ensure that the highest amplitudes of an input signal can be amplified in the same proportion as the lower amplitudes without distortion. However, there is an efficiency penalty for providing excess rail voltage to the amplifier for a given input signal. Excess rail voltage causes inefficiency and rapidly discharges power from a power source, such as a battery. Alternatively, if insufficient rail voltage is provided to the amplifier, then distortion may occur in the amplified signal.
Therefore, there is a need for an audio device that solves the problems described above. There is also a need for an amplifier containing an audio device that has improved efficiency, while preventing signal distortion.